The present invention relates to a method and to a device for acquiring and evaluating objects in the area surrounding a vehicle using a radar sensor.
German Published Patent Application No. 44 42 189 discusses a system for providing distance measurement in the area surrounding motor vehicles using sensors having transceiver units for the simultaneous transmission and reception of information. With the aid of the distance measurement, passive protective measures for the vehicle can be activated, for example in case of a front, side, or rear collision. With an exchange of the acquired information, a judgement of traffic situations can be carried out for the activation of corresponding triggering systems.
In addition, a distance measurement can be carried out or performed using what is known as a pulse radar system, in which a pulse carrier is sent out having a rectangular envelope of an electromagnetic oscillation, e.g. in the gigahertz range. This pulse carrier is reflected at the target object, and from the time from the transmission of the impulse and the impinging of the reflected radiation, it is possible to easily determine the distance to the goal, and (with limitations), using the Doppler effect, the relative speed of the target object as well. Such a measurement design is, for example, discussed in the textbook by A. Ludloff, xe2x80x9cHandbuch Radar and Radarsignalverarbeitung,xe2x80x9d pages 2-21 to 2-44, Vieweg Verlag, 1993.
The design of such a known radar sensor is constructed in such a way that the radar pulses reflected at the respective target object travel to a receiver via antennas, and there they are mixed with the time-delayed pulses provided by the pulse production system. After a low-pass filtering and analog/digital conversion, the output signals of the receiver are supplied to an evaluation unit.
To control reliably the above-mentioned passenger protection systems in a motor vehicle, a multiplicity of radar sensors may be required for the individual conflict situations in the area surrounding the motor vehicle. For example, a collision early-recognition (pre-crash recognition) system enables an early acquisition of an object that represents a danger for the vehicle occupants in case of a collision. In this manner, protective systems, such as an airbag, seat-belt tensioner, or side airbag, may be activated at the proper time to achieve the greatest protective effect.
For proper triggering of these safety systems in the motor vehicle, knowledge of the relative velocity (speed) between the motor vehicle and one or more targets (e.g., vehicles traveling ahead or other obstacles) before and during an anticipated collision, and of the expected time of the collision, may be of great importance.
Using a radar sensor of the type mentioned above, methods can be carried out or performed, for example, using a pulse radar sensor or what is known as an FMCW radar sensor, that enable an acquisition and evaluation of the relative velocity. Such an FMCW radar device is discussed in European Patent Application No. 0 685 930.
For example, at successive times, distance values can be measured and differentiated with respect to time. In this way, one obtains the values for the instantaneous relative velocity between the target and the radar sensor. Through a double differentiation of the distance values, the values for the acceleration relative to the target may be obtained. Using a different method, the difference between the transmitted oscillator frequency of the radar sensor and the frequency of the signal reflected and received from the target can be produced, and what is referred to as the Doppler frequency can be evaluated.
From the values measured in this way, the time until the collision, and, with the use of a plurality of spatially distributed sensors, the components orthogonal to the front of the vehicle of the relative velocity or acceleration, and the location of the collision, may be calculated. Using the instantaneous values of the acceleration, the corresponding values for the time of the collision can then be extrapolated.
In this context, a high degree of measurement precision may be important, in particular given targets having a low reflection cross-section and given high disturbing signal portions in the velocity range that is to be evaluated for the respective application (e.g., triggering of the seat-belt tensioner or changing over of the stages of the airbag). Here, previous measurement methods are based on a constant length of the acquisition area (region) in the area being monitored, and/or a constant distance from this area to the radar sensor.
An exemplary method and device for acquiring and evaluating objects in the area surrounding a vehicle using a radar sensor of the type indicated above may be advantageously provided in that in a monitoring area (monitored area or monitored region), the acquisition of the target object takes place within a virtual barrier, known as a xe2x80x9crange gate,xe2x80x9d that can be modified in its distance from the vehicle and in its length.
After an evaluation of the targets acquired using a radar sensor with respect to their potential risk, the distance and the velocity, as well as, if necessary, the acceleration relative to the target object are measured. Using the adaptive construction of the dimensions of the virtual barrier according to the present invention, the measurement process may be advantageously optimized with respect to measurement precision, locus resolution, and the signal/noise ratio.
In an exemplary method according to the present invention, using a transmission signal of a pulse radar sensor, the received signal reflected from the target object is evaluated in at least two receive channels in such a way that different locus resolutions and different dimensions with respect to distance and length of the virtual barrier are achieved.
In a first receive channel, the received signal for the acquisition of the distance of the target objects is processed using a reference signal having a fixedly set pulse duration xcfx84s, corresponding to the transmission signal. In a second receive channel, the receive signal may be advantageously processed using a reference signal having a modifiable pulse duration xcfx84R, either for the measurement of the distance with a modifiable locus resolution or for setting the length xcex94xVB of the virtual barrier.
The exemplary method according to the present invention may be executed in particularly advantageous fashion as described in the context of the mathematical relationships of FIG. 3.
An exemplary device for performing the exemplary method according to the present invention includes a pulse radar sensor, having in particular a first receive channel for distance measurement and a second receive channel for setting the virtual barrier in the sense previously described.
The adaptive setting of the length xcex94xVB≈xcex94xmess of the virtual barrier enables, in a relatively simple manner, an optimization of the value for xcex94xmess with respect to measurement precision, locus resolution, and the signal/noise ratio. Given a high velocity relative to the vehicle, target objects having a low reflection cross-section are recognized, because in this case a greater value is used for the length of the virtual barrier.
The setting of a distance of the virtual barrier to the radar sensor that is as small as possible, with as low a value as possible for the length xcex94xVB may have the following advantages over larger values for the distance:
If the vehicle and the target object move past one another, the probability that a target object moves through the virtual barrier with a high relative velocity is lower; in this way, false triggerings or false measurements may become less probable.
The signal/noise ratio is larger given a smaller distance of the target object to the radar sensor, and also allows the detection or measurement of target objects having a low reflection cross-section.
In addition, it may also be advantageous that low target objects that are not supposed to be recognized, such as for example curbstone edges, are not acquired by the radar sensor dependent on their height, given lower values for xcex94xVB or xcex94xmess, the constructive height of the radar sensor, and the horizontal opening angle of the radar transmission antenna or of the reception antenna.
With the exemplary method according to the present invention, the time of the measurement can advantageously be selected relative to the time of the expected collision of the vehicle with the target object, corresponding to the requirements of the applications.
The various features described herein can respectively be realized individually or in subcombinations.